KVM: x86: Add a helper to check for a legal GPA

mainline inclusion
from mainline-v5.12-rc1
commit 4bda0e97
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I7S3VQ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4bda0e97868a95553ba71d87f593756e1ffd654b



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Add a helper to check for a legal GPA, and use it to consolidate code
in existing, related helpers.  Future patches will extend usage to
VMX and SVM code, properly handle exceptions to the maxphyaddr rule, and
add more helpers.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210204000117.3303214-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>